Chromatic dispersion is a particularly important characteristic of optical fibers due to the distortion effects it causes on the optical signals that propagate in fibers for telecommunications.
Such a phenomenon is, as is well known, linked to the different group velocities with which various spectral components of optical signals propagate in a fiber, for instance between an optical transmitter and receiver or a first and a second end of the fiber.
The spectral components of an optical signal, due to the different group velocities, reach the receiver at different times and determine a distortion of the received signal both in analogue transmission systems, and in digital transmission systems.
The chromatic dispersion of optical fibers, as is well known, is determined mainly by two factors, the dispersion characteristics of the material whereof the fibers are made (dependence of the refractive index of the material on frequency), and the very nature of the propagation of an optical signal in a waveguide (the so-called “waveguide dispersion”).
The chromatic dispersion of optical fibers can be suitably controlled by a careful design of the profile of the refractive index of the fibers. In fact, today, optical fibers are manufactured with chromatic dispersion characteristics specifically studied to meet the requirements of the most sophisticated optical transmission systems.
As these requirements become ever more stringent, in particular for transmission systems with bit rates of 10 Gbit/s or higher, the need also increases to “characterize” the CD of the optical fibers with accuracy, not only in the factory during the product qualification stage, but also in field, for new or less recently installed systems.
Such “characterization” is indispensable in order to design and install new-generation transmission systems (with bit rates of 10 Gbit/s or higher) on recent fibers or to verify, in case of less recent systems, the possibility of supporting a higher transmission capacity.
Today, the chromatic dispersion of optical fibers is measured by means of at least three different, consolidated techniques, as described, for instance, in the ITU-T Recommendation G.650. Amongst such techniques, one of the most commonly used is is the so-called Phase Shift (or PS) technique. It consists of measuring the phase shift introduced by the optical fiber segment being measured on a sinusoidal signal that modulates an optical carrier which is made to propagate in the fiber itself. The measurement is repeated at different wavelengths of the aforesaid optical carrier and for each of them the group delay of the modulating sinusoidal signal, which is proportional to the aforesaid phase shift, is calculated.
An apparatus or instrument for measuring chromatic dispersion is, for instance, described in U.S. Pat. No. 6,313,934.
This document describes, inter ails, a methodology for measuring chromatic dispersion wherein the phase shift of the sinusoidal signal, that modulates the optical carrier, introduced by the fiber segment is measured by synchronizing, by means of absolute timing systems obtained from a GPS (Global Positioning System), appropriate devices for generating and measuring the sinusoidal signal, positioned at the ends of the fiber itself.
The prior art method entails the need to simultaneously access the two ends of the fiber to be characterized, to apply appropriate instrumentation to the two ends of the fiber and to use an absolute reference system with which to synchronize the instrumentation at the two ends of the fiber.